A nonaqueous electrolyte secondary battery in which charge and discharge is conducted by moving Li ions between a negative electrode and a positive electrode has recently been proposed for use as a large size power storage device for an electric vehicle (EV), hybrid electric vehicle (HEV), or a stationary power generation system such as a solar power generation system, in consideration of energy issues, environmental issues, and the like.
Such a nonaqueous electrolyte secondary battery is required to have properties such as a useful lifetime longer time than that of small-sized nonaqueous electrolyte batteries used for a cell phone, laptop computer, or the like, and entail a lower risk of combustion or explosion in the unlikely event of an accident.
It is known that in a nonaqueous electrolyte battery using a carbon negative electrode, carbonates, which are structural components of an electrolytic solution, are reductively decomposed on a surface of a negative electrode active material to form a coating film called “SEI” (solid electrolyte interface) upon initial charge and discharge. After the formation of the SEI, it is possible to prolong the life of the battery, because decomposition of a solvent on the surface of the negative electrode active material is inhibited.
Furthermore, the life of a battery greatly depends on the condition of the SEI coating film, and it is considered that an SEI coating film which is formed to be thinner and denser is generally better.
In a lithium ion secondary battery using the carbon negative electrode, however, a solvent is positively decomposed upon initial charge and discharge, and thus a large amount of SEIs are formed. If a large amount of SEIs are formed, a resistance of the nonaqueous electrolyte battery may be increased.
The term nonaqueous electrolyte battery includes, for example, nonaqueous electrolyte batteries that use a lithium titanium composite oxide as a negative electrode material, in addition to lithium ion secondary batteries that use a carbon negative electrode.
In the lithium titanium composite oxide, the size and the structure of the crystal lattice thereof are hardly changed upon absorption and release of Li ions, and thus it is known to be a material with excellent cycle stability. In addition, if the battery is shorted by accident, the site of the short quickly enters a high-resistance discharged state, and thus abnormal heating of the battery, caused by a large current flow, can also be prevented. When the lithium titanium composite oxide is used as the active material, accordingly, a battery which is excellent in both the cycle stability and the safety can be produced.
Furthermore, in a nonaqueous electrolyte battery using the lithium titanium composite oxide as a negative electrode material, rapid charging and discharging can be stably conducted.
In the battery using the lithium titanium composite oxide as the active material, however, it is more difficult to form SEI upon the initial charge and discharge, compared to batteries using the carbon negative electrode. It can be considered that this is caused because in the lithium titanium composite oxide, a potential at which the absorption and release of Li ions occurs is higher than that in the carbon negative electrode, and thus a potential, at which reducibility satisfactory for forming the SEI is generated, is not attained in the composite oxide. The battery using the lithium titanium composite oxide has, accordingly, an active material with a high cycle stability and a low initial increase of resistance, but if no measures are taken, an increase of resistance or gas generation is caused due to decomposition of an electrolytic solution during cycles or storage, and to development of a coating film due to the decomposition of the electrolytic solution may result.